EP1091369A2

EP1091369A2 - Low profile transformer and method for making a low profile transformer

Info

Publication number: EP1091369A2
Application number: EP00308677A
Authority: EP
Inventors: Galliano Riccardo Busletta; William Lonzo Woods, Jr.; Matthew Anthony Wilkowski; Robert Joseph Roessler
Original assignee: Lucent Technologies Inc
Current assignee: Nokia of America Corp
Priority date: 1999-10-07
Filing date: 2000-10-03
Publication date: 2001-04-11
Also published as: JP2001143940A; EP1091369A3

Abstract

A low profile electrical transformer (10) comprises at least two electrical windings (14) and a ferromagnetic base member (12). The base member is an integral construction presenting a core structure and a plurality of termination structures. The electrical windings are arrayed about the core structure and present a plurality of lead structures which are attached with the termination structures for electrical attachment in a circuit. A method for making the low profile transformer having an electrical winding structure including at least electrical windings and a plurality of termination structures; and a base member having a plurality of termination sites; comprises the steps of: (a) assembling the electrical winding structure and the base member; and (b) during the assembling, attaching the plurality of electrical termination structures with at least some of the plurality of termination sites to present a plurality of electrical termination loci for connection within an electrical circuit.

Description

Background Of The Invention

The present invention is directed to electrical transformers, and especially to electrical transformers presenting a low profile in their assembled state. The invention also contemplates a method for making a low profile transformer.
In today's industrial environment, the applications for using electrical transformers are just as numerous as has been the case in the past. In fact, in some industries there is even more demand for electrical transformers for isolation, for power supplies, and for other well-known uses. The devices employing transformers, moreover, have been evolving to smaller and smaller packages in recent years. For example, the portable telephone has shrunk from a bag phone or a car phone with the electronics mounted in the automobile trunk, to portable phones the size of a pack of cigarettes, or smaller. All of this shrinkage requires that the components used in manufacturing the smaller devices must themselves be smaller. Transformers have been no exception.
Conventional transformers are fabricated by winding wire on a plastic bobbin, or header, and terminating those windings at the bobbin. Thereafter, the bobbin-winding unit is assembled with a magnetic core to enhance the transformer's electrical properties, as is well known in the art. However, it has been observed that such use of a bobbin, or header, has a significant impact on the height of the transformer product. Up to fifty per cent of the height of the finished transformer can be attributable to the bobbin or header being used to support the windings.
One approach to producing a low profile transformer for a smaller device has been to flatten the transformer ― bobbin, core, and all. This solution results in drawbacks. The flattened configuration results in longer wire path length for each turn of the windings, resulting in greater direct current (DC) resistance as compared to a non-flattened transformer with the same number of turns. Thus, the performance of the flattened transformer can be inferior to the non-flattened design for a like number of turns, and more power is dissipated during operation.
Another shortcoming of the flattened transformer design is that the part occupies a larger "footprint" when incorporated in a circuit. Such a larger footprint results in the transformer taking up more room in a circuit, thus defeating the purpose of flattening the part in the first place: to accommodate smaller circuit applications for smaller products.
Another problem has been encountered as designers strive to make transformers smaller: the cores become physically less robust and are prone to fractures. Yet another consequence of smaller transformers occurs when the decrease in size is achieved by reducing the cross-section of the transformer core. Operating a transformer with a lesser cross-section in its core causes greater core loss power dissipation and deterioration of initial electrical properties, such as inductive flux-carrying capacity. The reduced cross section, or area, of the winding also reduces inductance and limits the available power the magnetic device can deliver.
There is a need for a transformer having a low profile which does not sacrifice efficiency to achieve its compact configuration. There is also a need for a transformer having a low profile which does not need to occupy extra real estate in circuitry in order to achieve its lower height. Additionally, it is important that achieving a low profile in a transformer does not sacrifice robustness of the part or increase core losses of the part.

Summary Of The Invention

The preferred embodiment of the present invention is a low profile magnetic device, preferably in the form of an electrical transformer, comprising at least two electrical windings and a ferromagnetic base member. The base member is preferably an integral construction presenting a core structure and a plurality of termination structures. The electrical windings are arrayed about the core structure and present a plurality of lead structures which are attached with the termination structures for electrical attachment in a circuit. The invention also includes a method for making the low profile magnetic device. In its preferred embodiment the method of the present invention for making the low profile transformer having an electrical winding structure including at least electrical windings and a plurality of termination structures; and a base member having a plurality of termination sites; comprises the steps of: (a) assembling the electrical winding structure and the base member; and (b) during the assembling, attaching the plurality of electrical termination structures with at least some of the plurality of termination sites to present a plurality of electrical termination loci for connection within an electrical circuit.
In its preferred embodiment, the apparatus of the present invention has an insulative layer on the base member. In its most preferred embodiment, the insulative layer is a coating of insulative material, such as an epoxy material or paralene.
Thus, the preferred embodiment of the present invention includes a ferromagnetic core and bondable wire wound rings or a planar sheet of copper foil. The wire round rings or sheet copper foil are assembled directly with the core without any intervening bobbin or header. The wire wound rings are insulated from each other and have their ends stripped to expose the wire to facilitate electrical connection. In the embodiment including sheet copper foil, the copper foil preferably has an insulating spacer above and below the copper foil. The only physical connection between the wire wound rings or foil and the ferromagnetic base, or core, in the preferred embodiment is the winding of the stripped wire ends or foil about discrete mounting posts of the base member. There is no electrical connection between the wire wound rings or foil and the core because, as mentioned earlier, the core is insulated, preferably with an insulating coating of paralene. This leaves exposed wire or foil wrapped about mounting posts, and the unit can be surface mounted upon a substrate, such as a printed wiring board, by such connection technology as infrared reflow soldering, vapor phase reflow soldering, wave soldering, conductive epoxy, or other mounting methods.
A lid, preferably ferromagnetic, may be applied to improve the physical integrity of the package, as well as to enhance the flux integrity of the transformer product.
It is, therefore, an object of the present invention to provide a low profile transformer which does not sacrifice efficiency to achieve its compact configuration.
A further object of the present invention is to provide a low profile transformer which does not need to occupy extra real estate in circuitry in order to achieve its lower height.
Yet a further object of the present invention is to provide a low profile transformer which does not sacrifice robustness of the part or increase losses of the part.
Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings, in which like elements are labeled using like reference numerals in the various figures, illustrating the preferred embodiments of the invention.

Brief Description Of The Drawings

FIG. 1 is an exploded view of the preferred embodiment of the present invention in unassembled form.
FIG. 2 is a top plan view of the preferred embodiment of the present invention in assembled form with the top removed.
FIG. 3 is a top plan view of the preferred embodiment of the present invention in assembled form with the top installed.
FIG. 4 is a side view of the preferred embodiment of the present invention in assembled form, taken along section 4 ― 4 in Fig. 3.
FIG. 5 is a flow diagram illustrating the preferred embodiment of the method of the present invention.

Detailed Description Of The Preferred Embodiment

FIG. 1 is an exploded view of the preferred embodiment of the present invention in unassembled form. In Fig. 1, a transformer assembly 10 includes a base member 12, an electrical winding structure 14, and a cover 16.
Base member 12 includes a centrally situated magnetic flux concentrating structure, or core 18 extending from a foundation 20. Also extending from foundation 20 are a plurality of substantially similar support posts 22, 24, 26, 28. An annular channel 13 is thus established intermediate core 18 and support posts 22, 24, 26, 28. Preferably annular channel 13 is appropriately dimensioned to receive electrical winding structure 14 during assembly of transformer assembly 10. Each support post 22, 24, 26, 28 includes an integrally formed pair of termination sites. Thus, support post 22 includes termination sites 30, 32; support post 24 includes termination sites 34, 36; support post 26 includes termination sites 38, 40; and support post 28 includes termination sites 42, 44. An alignment, or orientation ridge 46 is also included extending from foundation 20 to aid in orientation or alignment of the transformer after assembly. Preferably, base member 12 is constructed as an integral piece of ferromagnetic material. Base member 12 preferably is insulated from other components of transformer 10. In its most preferred embodiment, base member 12 is coated, or encapsulated, with a deposited insulative coating, such as an epoxy or paralene coating. In an alternate embodiment, base member 12 may have only areas proximate to termination sites 30, 32, 34, 36, 38, 40, 42, 44 coated with an insulating material.
The present invention can be employed for air-core transformers. In such an embodiment, base member 12 need not be constructed of ferromagnetic material, and preferably would not be constructed of such material. Further in such an embodiment, core 18 may be used merely as a positioning structure to aid in orienting electrical winding structure 14 during assembly, or core 18 may be eliminated.
Electrical winding structure 14 includes three distinct wire coils substantially concentrically wound in a cylindrical winding 15 having a central aperture 17. Discrete wire coils are not shown in detail in Fig. 1 because the winding of multiple coils in substantially concentric fashion is known in the art, and any such known winding method may be employed in the winding of electrical winding structure 14. In fact, there may be any amount of two or more discrete coils in electrical winding structure 14, depending upon the nature of transformer being constructed. The structure and method of the present invention contemplate inclusion of transformers having two or more windings.
Electrical winding structure 14 includes first coil electrical termination structures 50, 52, second coil electrical termination structures 54, 56, and third coil electrical termination structures 58, 60 for effecting electrical connection with the various coils included in electrical winding structure 14. The preferred form of electrical termination structures 50, 52, 54, 56, 58, 60 is bare wire solder-dipped leads.
Cover 16 is preferably constructed of material similar to the material used to fabricate base member 12. Cover 16 is proportioned to be substantially coextensive with the expanse of foundation 20. When transformer 10 is assembled, cover 16 is attached with base member 12 at support posts 22, 24, 26, 28.
In assembling transformer assembly 10, electrical winding assembly 14 is situated lying generally adjacent to foundation 20 in substantially concentric relation with respect to core 18 with core 18 within aperture 17, surrounded by electrical winding structure 14. Electrical termination structures 50, 52, 54, 56, 58, 60 are fastened with individual termination sites 30, 32, 34, 36, 38, 40, 42, 44, preferably by winding electrical termination structures 50, 52, 54, 56, 58, 60 about termination sites 30, 32, 34, 36, 38, 40, 42, 44, as will be described in greater detail hereinafter in connection with Figs. 2 ― 4.
FIG. 2 is a top plan view of the preferred embodiment of the present invention in assembled form with the top removed. In Fig. 2, electrical winding structure 14 is situated lying generally adjacent foundation 20. Aperture 17 is concentrically located about core 18; electrical winding structure 14 surrounds core 18. Electrical termination structure 50 is fastened with termination site 30, electrical termination structure 52 is fastened with termination site 32, and electrical termination structure 54 is fastened with termination sites 34, 36. All fastening is preferably effected by winding the respective electrical termination structure 50, 52, 54 about the respective termination site 30, 32, 34, 36. In the case of electrical termination structure 54, the fastening is made to both termination sites 34, 36 simply by winding electrical termination structure 54 about both termination sites 34, 36. Thus, termination sites 34, 36 are electrically common in the assembled transformer illustrated in Fig. 2. In similar fashion, electrical termination structure 56 is wound-fastened in electrical common with termination sites 38, 40. Electrical termination structure 58 is wound-fastened with termination site 42, and electrical termination structure 60 is wound-fastened with termination site 44. The fastening of electrical termination structures 50, 52, 54, 56, 58, 60 with termination sites 30, 32, 34, 36, 38, 40, 42, 44 may be effected in any combination of electrical termination structures and termination sites, or a combination of any subset of electrical termination structures and termination sites. The particular connections illustrated in Fig. 2 are for the purpose of illustration of the preferred embodiment of the invention only, and such connections are not intended to limit the scope of the invention in any way.
FIG. 3 is a top plan view of the preferred embodiment of the present invention in assembled form with the top installed. In Fig. 3, electrical termination structures 50, 52, 54, 56, 58, 60 are fastened with termination sites 30, 32, 34, 36, 38, 40, 42, 44 as described in connection with Fig. 2 above. In order to avoid prolixity in this specification, the detailed recitation of such fastening will not be repeated in connection with Fig. 3. Cover 16 is affixed to base member 12 (base member 12 is situated beneath-cover 16 and not visible in Fig. 3). A label 19 is illustrated as being affixed to cover 16, preferably by adhesive.
FIG. 4 is a side view of the preferred embodiment of the present invention in assembled form, taken along section 4 ― 4 in Fig. 3. In Fig. 4, cover 16 is shown resting atop support posts 22, 24, 26, 28 (only support posts 26, 28 are visible in Fig. 4). The preferred method for affixing cover 16 to base member 12 at support posts 22, 24, 26, 28 is by adhesive. Other affixation methods or materials are also acceptable. In the preferred embodiment of the invention illustrated in Fig. 4, there is a gap 62 between core 18 and cover 16 when cover 16 is installed. Gap 62 is advantageous because it establishes a gapped core construction so that higher DC (direct current) currents can be accommodated by transformer assembly 10 before saturation.
Fig. 4 further shows another feature of the present invention not readily observable from Figs. 1 ― 3. In its assembled form, transformer 10 presents contact areas suitable for surface mounting on a printed wiring board or similar substrate. In Fig. 4, those contact areas are identified as contact areas 64, 66, 68. Contact areas 64, 66, 68 present a generally rough planar area by the faces of windings of electrical termination structures 56, 58, 60. The preferred mounting method for attaching transformer 10 within an electrical circuit on a printed wiring board (not shown) is using infrared (IR) reflow solder technology. Such IR reflow solder mounting will fill in "rough" areas between windings of electrical termination structures 56, 58, 60 and firmly seatingly attach transformer 10 to a printed wiring board. Electrical termination structures 50, 52, 54 (not shown in Fig. 4) cooperating with termination sites 30, 32, 34, 36 (not shown in Fig. 4) will similarly provide contact areas and effect firm seating of transformer 10 during mounting upon a printed wiring board. Other connection technologies will similarly accommodate the structure of transformer 10 for providing solid seating during attachment within a circuit, such as vapor phase reflow solder, conductive epoxy, and similar mounting technologies.
FIG. 5 is a flow diagram illustrating the preferred embodiment of the method of the present invention. In Fig. 5, the process is begun by obtaining an electrical winding structure, such as electrical winding structure 14 of Figs. 1 ― 4, as indicated by block 70. Preferably the electrical winding structure obtained pursuant to block 70 includes at least two windings and a plurality of termination structures for electrically connecting with the respective electrical windings. Most preferably, the electrical winding structure is fabricated in a substantially rigid form to facilitate handling during assembly.
The method of the present invention also involves obtaining a base member, such as base member 12 of Figs. 1 ― 4, as also indicated by block 70. The base member may be cast or it may be prepared by other suitable manufacturing methods. For example, if the transformer to be fabricated is to be an air core transformer, then the base member may be molded. The base has a plurality of termination sites. Preferably, the base member is constructed as an integral piece of ferromagnetic material. The base member preferably is insulated from other components of the transformer. In its most preferred embodiment, the base member is coated, or encapsulated, with a deposited insulative coating, such as an epoxy or paralene coating.
Next, the base member and winding structure are assembled, as indicated by block 74. The winding structure is situated generally adjacent the base member. If the base member has a flux concentrating structure, the winding structure is situated with respect to the base member to advantageously interact with the flux concentrating structure during operation of the transformer. An example of such a flux concentrating structure is core 18 in the transformer illustrated in Figs. 1 ― 4. The plurality of electrical termination structures are attached with the plurality of termination sites, as indicated by block 76. Preferably the termination is effected according to a pattern required for proper mounting of the transformer in assembled form within a circuit, such as a circuit on a printed wiring board.
A cover is attached, as indicated by block 78, to complete the assembly of the transformer. A preferred technology for attaching the cover is to adhesively attach the cover to the base member in a manner capturing the winding structure intermediate the base member and the cover.
It is to be understood that, while the detailed drawings and specific examples given describe preferred embodiments of the invention, they are for the purpose of illustration only, that the apparatus and method of the invention are not limited to the precise details and conditions disclosed and that various changes may be made therein without departing from the spirit of the invention which is defined by the following claims:

Claims

A low profile electrical transformer comprising at least two electrical windings and a base member, said base member being an integral construction presenting a plurality of termination structures; said at least two electrical windings presenting a plurality of lead structures; said at least two electrical windings being arrayed upon said base member with said plurality of lead structures being attached with said plurality of termination structures for electrically discrete attachment in a circuit.
A low profile electrical transformer as recited in Claim 1 wherein said base member is a ferromagnetic base member is at least partially enclosed in an electrically insulating structure.
A low profile electrical transformer as recited in Claim 2 wherein said base member is enclosed in an electrically insulating structure.
A low profile electrical transformer as recited in Claim 1 wherein respective lead structures of said plurality of lead structures are attached with respective termination structures of said plurality of termination structures by winding said respective lead structures about said respective termination structures.
A low profile electrical transformer comprising; an electrical winding structure having at least two electrical windings and a plurality of electrical termination structures; a base member, and attachment loci; said electrical winding structure being attached with said base member to effect magnetic flux linking among said at least two windings; said at least two windings being attached at said attachment loci for electrically connecting said at least two windings in an electrical circuit.
A low profile electrical transformer as recited in Claim 5 wherein respective electrical termination structures of said plurality of electrical termination structures are attached with said base member by winding said respective electrical termination structures about said base member at a plurality of integral termination sites.
A low profile electrical transformer as recited in Claim 4, 5 or 6 wherein said base member is a ferromagnetic base member.
A low profile electrical transformer as recited in Claim 7 wherein said base member is at least partially enclosed in an electrically insulating structure.
A low profile electrical transformer as recited in Claim 3 or 8 wherein said electrically insulating structure is an insulating coating.
A method for constructing a low profile electrical transformer having an electrical winding structure and a base member; said electrical winding structure including at least electrical windings and a plurality of termination structures; said base member having a plurality of termination sites; the method comprising the steps of:

(a) assembling said electrical winding structure and said base member;

(b) during said assembling, attaching said plurality of electrical termination structures with at least some of said plurality of termination sites to present a plurality of electrical termination loci for connection within an electrical circuit.
A method for constructing a low profile electrical transformer as recited in Claim 10 wherein said base member is a ferromagnetic base member is at least partially enclosed in an electrically insulating structure.
A method for constructing a low profile electrical transformer as recited in Claim 10 wherein said base member is a ferromagnetic base member having a magnetic flux concentrating structure; said electrical winding structure being situated proximate said magnetic flux concentrating structure during said assembling.
A method for constructing a low profile electrical transformer as recited in Claim 12 wherein respective electrical termination structures of said plurality of electrical termination structures are attached with respective termination sites of said at least some of said termination sites by winding said respective electrical termination structures about said respective termination sites.